switch builtin_rands() to use boost::random per issue 234
experimental workaround for boost linker problems w random_device
remove boost::random_device, move min/max out of the random generator loop
fix typo
also use process ID to seed pseudo random number generator
clarify that its not nondeterministic, we are supporting older systems
where the C++/boost random_device stuff doesn't work.
boost random_device in particular has a lot of issues with old versions
not working, with -lboost_random not being installed by default, etc,
that complicate the build too much.
remove unneeded frand functions
Merge pull request #238 from openscad/issue234

Issue234